Rare earth elements (REEs) of low concentration are usually beneficial to plant growth, while they are toxic at high concentrations. The effects of treatment with lanthanum (La) (10 and 20 μM), cerium (Ce) (10 and 20 μM), and terbium (Tb) (10 and 20 μM) on seedling growth of alfalfa (Medicago sativa L.), which is one of the most important perennial leguminous forages in the world, were studied. The results showed that all three REE treatments quickened the germination of seeds. The length of shoot under La (20 μM) treatment was significantly shortened (P < 0.05). In addition, treatment with La, Ce, and Tb had a “hormesis effect” on root length. There was a significant decrease in chlorophyll content on treatment with the three REEs, and the degree of decline was in the order of La < Ce < Tb, under the same concentration. In vitro experiments and quantum chemical calculations were further performed to explain why the treatments with REEs reduced the chlorophyll content. In vitro experiments showed that La, Ce, and Tb treatments reduced the absorbance of chlorophyll, and the decrease followed in the order of La > Ce > Tb. Quantum chemical calculations predicted that the decrease in absorption intensity was caused by the reactions between La, Ce, Tb, and chlorophyll, which formed lanthanides-chlorophyll; and there were five types of stable lanthanides-chlorophyll. In conclusion, the decrease in chlorophyll content on treatment with REEs was caused by the change in chlorophyll structure.

The rare earth element lanthanum (La) has been proven to be beneficial for plant growth with a low concentration, and abscisic acid (ABA) which is a plant hormone also can regulate plant growth. In the present study, we investigated the germination and seedling growth of switchgrass (Panicum virgatum L.) under La (10 μM), ABA (10 μM) and La + ABA treatments. The results showed that La, ABA and La + ABA treatments could not significantly affect the germination and shoot length as compared to the control (P>0.05). However, La treatment increased the root activity and chlorophyll content, and ABA treatment enhanced root length and root activity (P<0.05). La + ABA treatments demonstrated that La could not significantly alleviate the promotion of ABA in root length, while ABA reversed the increase of chlorophyll content caused by La. The coregulation of La and ABA on chlorophyll content was further explored by in vitro experiments and quantum chemical calculations. In vitro experiments revealed that La, ABA, and La + ABA treatments reduced the absorbance of chlorophyll, and quantum chemical calculations indicated that the reduction of absorbance was caused by the reactions between La, ABA and chlorophyll. In vivo and in vitro experiments, together with quantum chemical calculations, demonstrated that both ABA and La could stimulate the production of chlorophyll, while they also could react with chlorophyll to produce La-monochlorophyll, La-bischlorophyll, and ABA adsorbed chlorophyll, which had lower absorbance. La + ABA treatment significantly decreased the chlorophyll content in vivo. This phenomenon was due to the fact that La and ABA formed LaABA compound, which markedly reduced the concentrations of ABA and La, and the effect of promoting chlorophyll production was overcome by the effect of reducing chlorophyll absorbance.